In the prototypical thermionic converter depicted in FIG. 1, a hot electrode 1 and a cold electrode 2 are mounted in parallel, facing one another. The hot electrode is connected to a heat reservoir through a hot finger or heat pipe 4, and the cold electrode is connected to a cold reservoir through a cold finger or heat pipe 5. At temperatures above 500 degrees Celsius, this can lead to considerable parasitic radiation losses if the reflectivity of either or both electrodes is much smaller than 100%. Furthermore, spacers 3 may be used to maintain the gap between the electrodes, increasing heat loss. The size of the gap is determined by engineering design considerations. A large gap of several hundred microns is relatively simple to manufacture and control, but requires additional means to reduce space charge effects. Also as a result of a large gap, positively charged ions form a neutral plasma with the emitted electrons, thereby preventing additional electrons from being repelled by a space charge. A smaller gap of less than a few micrometers reduces space charge issues but is difficult to maintain over the operating temperature range of the device. Further, if multiple devices 10 are combined in one module, the heat conduction through the electrical conductors 11 between the hot and cold sides of adjacent devices can also be a considerable loss factor. (FIG. 2)
In U.S. Pat. No. 3,169,200, a multilayer converter is described which comprises two electrodes, intermediate elements and oxide spacers disposed between each adjacent element. A thermal gradient is maintained across the device, and opposite faces on each of the elements serve as emitter and collector. Electrons tunnel through each oxide barrier to a cooler collector, thereby generating a current flow through a load connected to the two electrodes. One drawback is that the device must contain some 10.6 elements in order to provide reasonable efficiency, and this is difficult to manufacture. A further drawback results from the losses due to thermal conduction: although the oxide spacers have a small contact coefficient with the emitter and collector elements, which minimizes thermal conduction, the number of elements required for the operation of the device means that thermal conduction is not insignificant.
A further issue that arises with gap diodes is parasitic heat loss. Although a vacuum gap by itself is a perfect insulator, heat may flow from the hot side to the cold side through the spacers and the edge seals. Even if a material with low thermal conductivity is chosen for spacers and edge seals, the heat losses can be substantial if the substrates are chosen from a metal or semiconductor material due to the fact that the spacers and seals are very thin.
In WO03090245, a gap diode is disclosed in which a tubular actuating element serves as both housing for a pair of electrodes and as a means for controlling the separation between the electrode pair. In a preferred embodiment, the tubular actuating element is a quartz piezo-electric tube. In accordance with another embodiment of the present invention, a gap diode is disclosed which is fabricated by micromachining techniques in which the separation of the electrodes is controlled by piezo-electric, electrostrictive or magnetostrictive actuators. Preferred embodiments of gap diodes include Cool Chips, Power Chips, and photoelectric converters.
However, active elements such as piezo actuators may be complicated and costly, and thus the simplicity of a layered structure to provide separation of electrodes is desirable.
U.S. Pat. Nos. 2,915,652 and 3,041,481 describe thermionic converters in which the emitter and collector are arranged in a novel arrangement and which also comprise the addition of electric and magnetic fields so as to provide improved converter efficiency and functioning. The emitter and collector are arranged side by side and a third electrode is positioned above and facing the emitter and collector. A transverse magnetic field, in addition to the electric field provided by the third electrode curves and directs the emitted electrons to the collector. This arrangement counters the effects of space charge and minimizes the transfer of heat between the emitter and collector, thereby increasing efficiency. However, the abovementioned patents disclose no method for constructing such potentially valuable devices. Furthermore, such arrangements would be difficult to construct in a manner similar to a vacuum tube due to the relatively large thermal gradients within the device and during start up and shut down.
Avto Metals:
In what follows, “Avto Metals” is to be understood as a metal film having a modified shape, which alters the electronic energy levels inside the modified electrode, leading to a decrease in electron work function as described in the foregoing, and illustrated in FIG. 3 below.
In U.S. Pat. Nos. 6,281,514, 6,531,703 and 6,495,843 and WO9940628, a method is disclosed for promoting the passage of elementary particles at or through a potential barrier comprising providing a potential barrier having a geometrical shape for causing de Broglie interference between the elementary particles. In another embodiment, the invention provides an elementary particle-emitting surface having a series of indents. The depth of the indents is chosen so that the probability wave of the elementary particle reflected from the bottom of the indent interferes destructively with the probability wave of the elementary particle reflected from the surface. This results in the increase of tunneling through the potential barrier. When the elementary particle is an electron, electrons tunnel through the potential barrier, thereby leading to a reduction in the effective work function of the surface. In further embodiments, the invention provides vacuum diode devices, including a vacuum diode heat pump, a thermionic converter and a photoelectric converter, in which either or both of the electrodes in these devices utilize the elementary particle-emitting surface. In yet further embodiments, the invention provides devices in which the separation of the surfaces in such devices is controlled by piezo-electric positioning elements. A further embodiment provides a method for making an elementary particle-emitting surface having a series of indents.
In U.S. Pat. No. 6,117,344 and WO9947980, methods are described for fabricating nano-structured surfaces having geometries in which the passage of elementary particles through a potential barrier is enhanced. The methods use combinations of electron beam lithography, lift-off, and rolling, imprinting or stamping processes.
In U.S. Pat. No. 6,680,214, a method is disclosed for the induction of a suitable band gap and electron emissive properties into a substance, in which the substrate is provided with a surface structure corresponding to the interference of electron waves. Lithographic or similar techniques are used, either directly onto a metal mounted on the substrate, or onto a mold which then is used to impress the metal. In a preferred embodiment, a trench or series of nano-sized trenches are formed in the metal.
In WO03/083177, the use of electrodes having a modified shape and a method of etching a patterned indent onto the surface of a modified electrode, which modifies the electronic energy levels inside the modified electrode, leading to a decrease in electron work function is disclosed. The method comprises creating an indented or protruded structure on the surface of a metal. The depth of the indents or height of protrusions is equal to α, and the thickness of the metal is Lx+α. The minimum value for α is chosen to be greater than the surface roughness of the metal. Preferably the value of α is chosen to be equal to or less than Lx/5. The width of the indentations or protrusions is chosen to be at least 2 times the value of α. Typically the depth of the indents is =λ/2, wherein λ is the de Broglie wavelength, and the depth is greater than the surface roughness of the metal surface. Typically the width of the indents is >>λ, wherein λ is the de Broglie wavelength. Typically the thickness of the indents is a multiple of the depth, preferably between 5 and 15 times the depth, and preferably in the range 15 to 75 nm. FIG. 3 shows the shape and dimensions of a modified electrode having a thin metal film 40 on a substrate 42. Indent 44 has a width b and a depth a relative to the height of metal film 40. Film 40 comprises a metal whose surface should be as planar as possible as surface roughness leads to the scattering of de Broglie waves. Metal film 40 is given sharply defined geometric patterns or indent 44 of a dimension that creates a de Broglie wave interference pattern that leads to a decrease in the electron work function, thus facilitating the emissions of electrons from the surface and promoting the transfer of elementary particles across a potential barrier. The surface configuration of the modified electrode may resemble a corrugated pattern of squared-off, “u”-shaped ridges and/or valleys. Alternatively, the pattern may be a regular pattern of rectangular “plateaus” or “holes,” where the pattern resembles a checkerboard. The walls of indent 44 should be substantially perpendicular to one another, and its edges should be substantially sharp. The surface configuration comprises a substantially planar slab of a material having on one surface one or more indents of a depth approximately 5 to 20 times a roughness of the surface and a width approximately 5 to 15 times the depth. The walls of the indents are substantially perpendicular to one another, and the edges of the indents are substantially sharp.